This is a quirk of Sage. If you have discount rules in Sage, Sage will apply discounts pertaining to this rule on top of any discount which has been passed in when the order or invoice is edited.
At this stage, this is a limitation of Sage and the integration process. There are two options to prevent this:
1. Avoid using discount rules in Sage and manage this entirely in OpenCRM, or,
2. Do not apply the discount in OpenCRM and allow Sage to manage this.